1. Field of the Invention
This invention pertains to a surface acoustic wave filter element and a surface acoustic wave filter made by packaging it.
2. Description of the Related Arts
Recently, the use of cellular telephone sets and other cordless telephone sets is becoming more prevalent than ever before. It is estimated that the day will be soon when each person will carry a PHP (Personal Handy Phone) set, which will be a cordless telephone set of the next generation. When its network system is realized, its demand is expected to soar.
Cellular telephone sets have rapidly spread because they have become more compact in size and lighter in weight, thus light enough to be portable, in addition to a telephone having already been an indispensable communications tool. The receiver set of such a telephone set incorporates a filter for its high frequency circuit. The most recent cordless telephone sets adopts as such a filter a surface acoustic wave filter using a surface acoustic wave element, for further reductions in size and weight.
A surface acoustic wave filter has the following features:
(1) Because it can be manufactured by a photolithograph as with an LSI, it is highly reliable and can be put into use without an adjustment. Also, it is suitable for mass production. PA1 (2) Because the velocity of a surface acoustic wave is about ten-millionths of an electromagnetic wave, it can be easily made more compact and lighter.
Because a surface acoustic wave filter can handle frequencies higher than a crystal quartz filter, its applications are expected for communications of the next generation, such as satellite communications.
The filter of a transmitter or a receptor for use e.g. in such cellular telephone sets must have a minimum insertion loss for maximizing the reception and a large extra-band attenuation for suppressing an unnecessary signal.
FIG. 1 is a circuit diagram illustrating the conventional configuration of a surface acoustic wave filter.
More specifically, FIG. 1 shows the circuit configuration of a ladder shaped surface acoustic wave filter, in which surface acoustic wave resonators are connected in cascade in a ladder form.
The surface acoustic wave filter shaped in a ladder form operates as a band-pass filter. The surface acoustic wave filter comprises four sections 10 connected serially, each with a serial resonator S and a parallel resonator P connected in an L shape, with section 10 in the last stage connected to another parallel resonator P. In a first example, a design can be made such that the resonant frequency of serial resonator S is made equal to both the anti-resonant frequency of parallel resonator P and the center frequency of a band-pass filter. This makes a range from the resonant frequency of parallel resonator P to the anti-resonant frequency of serial resonator S the band-pass range of an input signal. In a second example, another design can be made such that, to enlarge a band-pass range, the anti-resonant frequency of parallel resonator P is set smaller than the resonant frequency of serial resonator S, and that the midpoint between the anti-resonant frequency of serial resonator S and the resonant frequency of parallel resonator P is set equal to the center frequency of a band-pass filter. This makes a band-pass range from a proximity of the resonant frequency of parallel resonator P to a proximity of the anti-resonant frequency of serial resonator S the band-pass range of an input signal.
FIG. 2 is a block diagram illustrating the conventional configuration of a surface acoustic wave resonator.
More specifically, FIG. 2 shows the basic configuration of serial resonator S or parallel resonator P forming an element.
This element forms, on top of a piezoelectric board (not shown), a driving electrode 20 generally called an IDT (Inter-Digital Transducer) and a reflector (a reflecting electrode) 30 on each side of the driving electrode 20.
The driving electrode 20 is formed by two facing comb-shaped electrodes 20U and 20L locked into each other. The wave length .lambda. of a surface acoustic 20 wave excited on the surface of a piezoelectric board via the driving electrode 20 and the distance between two electrodes of such a driving electrode 20 (grating cycle) l.sub.1 has the following relation: EQU l.sub.1 =.lambda./2
Also, the reflector 30 is of a short strip type in which a plurality of electrode strips 31 are formed in parallel at the interval of a grating cycle l.sub.2. The grating cycle l.sub.2 is set equal to a half of the wave length .lambda. of a surface acoustic wave.
That is, l.sub.1 =l.sub.2.
A surface acoustic wave resonator (shown in FIG. 2) is an energy containment type resonator for obtaining a resonance characteristic by containing a surface acoustic wave in itself. It has the driving electrode 20 excite a surface acoustic wave on a surface of the piezoelectric board, has two reflectors 30 reflect a surface acoustic wave progressing on both sides of the driving electrode 20.
For improving an energy containment efficiency, such a resonator sets a lengthwise distance (hereafter called an aperture length) L2 of an aperture 32 of the reflector 30 equal to a lengthwise distance (hereafter called an aperture length) L1 of an aperture 22 formed between an upper comb-shaped electrode 20U and a lower comb-shaped electrode 20L of the driving electrode 20.
For improving the characteristics as a band-pass filter, i.e. for increasing its extra-band attenuation, of the surface acoustic wave filter whose configuration is shown in FIG. 1, it is necessary to increase the number of ladder stages, i.e. the number of the sections 10 connected in cascades. However, since this will increase the number of resonators, the overall size of the element tends to become larger, thereby presenting an obstacle in an effort to make the hardware more compact and lighter.